A configuration of transistors known as a cascode configuration provides an amplifier with a low input capacitance and a high input resistance. A simplified example of such a configuration, referred to generally as a cascode transistor, is shown in an amplifier in FIG. 1. The common-emitter transistor receives an input signal on its base and provides current gain to the amplifier. The common-base transistor has an emitter coupled to the collector of the common-emitter transistor and a collector that outputs the amplified signal. It provides the voltage gain of the cascode structure. The gain of the cascode configuration is the product of the current gain of the common-emitter transistor, and the voltage gain of the common-base transistor.
In order to obtain a conventional cascode amplifier with increased power capabilities, the common-emitter and common-base transistors of FIG. 1 are each replaced with a plurality or bank of like transistors connected in parallel as shown in FIG. 2. In this configuration each "transistor" has an increased power-handling capability due to the sharing of current flow between associated parallel transistors. The respective bases, collectors and emitters of each bank of transistors are connected, providing for common biasing and equal performance so long as the individual transistors within a bank operate substantially identically and share the current equally.
However, if there is an imbalance between the power-conducting common-base transistors, the one conducting more current becomes warmer. This in turn causes it to conduct more current, reducing the current conducted by the other parallel transistors. This reaction accelerates, a process known as thermal runaway, until secondary breakdown occurs in the transistor. Gallium arsenide is the semiconductor used almost exclusively in microwave applications due to its higher electron mobility and substantially reduced losses. Thermal runaway is more severe in a gallium arsenide transistor due to its reduced thermal conductivity as compared with Silicon.
What is needed therefore is a high power amplifier with the characteristics of a cascode configuration of gallium arsenide transistors that is safely operable at microwave frequencies.